a) Field of the Invention
The invention relates to a valve system for load-independently controlling a plurality of simultaneously operated hydraulic consumers or actuators (under any condition of load), as defined in the classifying portion of claim 1.
b) Background Art
A valve system of this type is disclosed in DE 36 34 728 A1 and is used to keep the volume flow rate to the actuator constant regardless of variations in load conditions. The feed lines which connect each actuator to a common pump supply have inserted or incorporated a directional valve, i.e. a way-valve for directional and speed control,and downstream of such valve also a throttling valve whose piston is acted upon to the closing position by the peak load pressure, i.e. the respective highest load pressure in any one of the actuated consumers and by a spring and in the reverse direction by fluid pressure exerted downstream of the way-valve or directional valve.
The instantaneous maximum load or load pressure of any one of the plurality of actuated consumers at a time is sensed by a common line, called load sensing line, and is transmitted to a regulating device of the variable displacement pump, i.e. a pump having a variable displacement capability to react to changes in fluid demand. With this particular arrangement it is possible to keep the pressure difference across the way valve orifice functioning as a metering throttle constant regardless of variations in load pressure and to simultaneously ensure that the respective fluid flows to each one of the actuated consumers under varying load conditions are kept at a constant ratio or proportion. The way-valve or directional valve is thus acting as a fluid metering device. In the described valve arrangement the ratio of the regulated fluid flows is even maintained constant if peak demands cannot be supplied, i.e. if the volume flow rate is insufficient to feed all actuators. In this case all partial flow rates to the individual actuators are reduced in proportion to the fixed ratio.
The circuitry of this known valve arrangement is shown in the schematic of FIG. 1, to which reference will be made in the following. The variable volume displacement pump which picks up fluid from a reservoir T is referenced with numeral 1. The pump is controlled by a pump regulator 12 in order to regulate the pump discharge in dependance of fluid pressure in a load sensing line LS. The consumer feed line is referenced 7, it being understood that each actuator is connected to a branched-off feed line 7.sub.1 to 7x. In FIG. 1 only one actuator is depicted by the dash-dotted box V. Each one of the branched-off feed lines incorporates in sequence a flowmeter 6 and a so-called compensator 3 in the form of a compensated flow control valve, i.e. a 2-directional flow regulating valve having a metering or control edge 4. To bias the valve or the regulating spool into the closing direction, a spring 13 plus the actual load pressure in a load sensing line 10 act in closing direction while in the reverse direction fluid pressure in the feed line 9 downstream of flowmeter 6 acts to open up the valve.
The known valve system uses elements of equal nominal size or caliber thus rendering the control of the individual pilot valves or flowmeters rather complex. In addition the known arrangement renders it difficult to adjust the system on demand to special functions of the control circuit or block without being forced to replace the pistons in the various valves.
In order to adjust the volume flow rate across valves with smaller sized or calibered pistons German OS 34 28 403 proposes a valve arrangement in which the way-valve or directional valve in the form of a flowmeter is designed as a two-stage controlled orifice valve acting as a flow amplifier. Again each actuator is associated with a compensator which is biased at one end by fluid pressure in the supply line downstream of the directional valve or pilot valve and at the other end by a spring and by the pressure at the respective consumer or actuator.
In this type of flow regulator or regulating control valve the pressure drop at the metering throttle is determined by the setting of the pressure compensator spring. When operating a plurality of consumers in parallel and assuming that in this case the demanded volume flow rate exceeds the feed capacity of the pump the respective consumer which is subject to the highest load pressure will be discriminated in favour of the other actuators operating at lower load pressure, respectively. In practice, fluid flow to the actuator with the highest load pressure will gradually decrease thus slowing down actuator motion until stopping occurs. Accordingly, the system does not provide for reducing fluid flow in equal proportion in the event of lack of fluid supply to the actuators. A further disadvantage of the known arrangement is the need to use a large-caliber spool in the main stage of the directional valve and in the compensator. Because both the directional valve main stage and the pilot valve stage use 4-way, 3 position spools, also in this case special functions of the control circuit can only be accomplished by changing the spools.
To provide greater flexibility with respect to the control functions of the control circuit, European Patent 0 079 870 B1 discloses a valve system in a so-called "detached construction". The compensator in this case is a small-sized or small-calibervalve. The compensator holds a secondary fluid flow constant which also passes through a small-sized flowmeter which is also of small-caliber. However, as the pressure difference or pressure drop across the flowmeter orifice is also determined by the setting of a spring that acts in opening direction of the compensator, and provided all consumers are operated in parallel and instantaneous pump supply is deficient, the consumer subject to the respective highest load pressure will be discriminated.
An object of the invention is to improve a valve system for the purposes described by employing an arrangement capable of performing special functions without requiring expensive spool replacements, yet simplifying operation of the main valves which regulate the fluid flow to the consumer while at the same time ensuring that the consumer subject to maximum load in the event of parallel operation of several consumers and insuffient pump supply will not be discriminated.
According to the invention this object is attained by a valve system for controlling a plurality of simultaneously actuated by hydraulic consumers or actuators, in which each consumer feed line is connected across a compensating flow control valve or compensator to be opened by displacing the spool under the action of fluid pressure downstream of a flow meter against the opposing resistance offered by a spring and the actual maximum or highest load pressure of the consumer. The highest load pressure is simultaneously transmitted to a regulating device of a variable displacement pump which supplies the consumers. The compensator and the flow meter are inserted in a secondary feed line which carries a fraction of the fluid flow feed to the consumer. The fluid flow feed is regulated by a metering edge of larger cross section and adapted to participate in the control movements of the compensator. The metering edge is supplied by valve means capable of reducing the fluid pressure in the main feed line to the pressure level downstream of the flow meter.
According to the invention, the control edge or orifice, respectively, of the valve is associated with an amplifying stage which constitutes the sole large-sized or large-caliber element in the hydraulic circuitry through which fluid passes on actuation of a consumer. Since only a partial flow is directed across the adjustable flowmeter or throttle and the control orifice, these elements can be designed as pilot control elements with small caliber. By setting a fixed ratio, for example 10:1, for the control orifice areas provided at the control edge on one end and at the amplifying stage on the other end, in the line upstream of the metering edge and the amplifying stage the volume flow rate passing through the main line is higher in proportion to the fixed orifice area, provided that fluid pressures in these lines are equal. Downstream of the metering edge or the control orifice of the amplifying stage both partial flows may merge to make up a regulated total volume flow rate. Because the larger control orifice is supplied by a valve capable of reducing pressure in the main line to the fluid pressure downstream of the flowmeter, with a constant relationship between the areas of the control edge and of the metering orifice of the amplifier the resultant transmission ratio at the compensator or at the adjustable flow control valve will be constant, because an equal fluid pressure is held up upstream of the compensator both in the main line and in the secondary line. For the many individual consumers to be supplied only a single pressure reducing valve in the main line is required, whereby the complexity of the system can be minimized. By indirectly adjusting a relatively high volume flow rate using a valve, i.e. an adjustable small-sized or small-caliber flowmeter, it will be possible to actuate the valve of large orifice area not only by hydraulically operating means, but also by mechanical, electrical, electro-hydraulical or pneumatical means. For example, the actuating force to control the flowmeter or a respective pilot valve, respectively may thus be supplied by directly working electrical devices such as solenoids or the like.
By dividing the volume flow rate to be regulated into a main and a secondary flow path additional control functions of the valve arrangement or of the control circuit incorporating the present valve system, pressure limites functions, deceleration valve functions and functions for preventing cavitation at low pressure operation of the consumers in a deceleration mode may be accomplished with valves of small piston area. This offers the special advantage of designing the valve system as a so-called "detached system" which eliminates the need of troublesome piston replacement in order to comply with the above mentioned auxiliary functions. For example, all way-functions of a 4-way valve may be controlled singly or in unison thus providing routing connections that would otherwise only be possible by changing the large-sized valve pistons.
The valve means that reduces the fluid pressure in the main feed line to the pressure downstream of the flowmeter is advantageously selected from pressure reducing valves. Because fluid pressure in the bypassed or secondary line downstream of the flowmeter in the balanced state of the compensator is equal to the maximum load pressure in the system plus a small amount necessary to open up the compensator as determined by the spring setting, the same rise in pressure must be available or assured at the pressure reducing valve. This is accomplished by appropriately matching the compression spring to the closing spring of the compensator.
The inventive concept to regulate the primary fluid supply regardless of load pressure variations, i.e. load-independently by means of the regulating motions of a compensator which is interposed in a secondary or bypass line, may also be useful when employed in the return line of a consumer. With such a valve arrangement the return side of a consumer may be controlled in a similar manner regardless of the load pressure, i.e. load-independently. In this instance, too, the control orifices are rigidly connected with one another, the ratio between the respectively control edge areas being predetermined. The previously described advantages in circuit design with respect to the provision of special functions as required by the control circuit equally apply to the return side of a consumer.
One improvement employs simple means to prevent a load from falling. When the pilot valve is not actuated or working, due to the throttle valve pressure build-up in the main return line as well as in the secondary return line is ensured thus acting to displace the compensator piston to the closing position.
Further improvements are incorporated in the valve system especially aimed at providing added auxiliary functions, yet protecting the valve system when operating the consumers at low fluid pressure, which would render the control system liable to cavitation.
The circuitry ensures that for lowering the consumer a drain port in the piston of the deceleration valve begins only to open when the pressure at the inlet increases sufficiently to move the spool against the force exerted by the spring and the pressure in the return line. Apart from the effect of lowering the load on the consumer regardless of variations in load pressure, it is additionally prevented that a lack of fluid supply to the consumer line is caused due to the fact that by acting of a heavy load on the consumer a greater amount of fluid would be dumped to the reservoir than is supplied to the consumer.
The deceleration valve is advantageously designed as a 2-way, 2 position proportional valve and thus contributes to minimize circuit complexity.
A further advantageous embodiment provides for controlling the return flow rate regardless of variations in load pressure. This version differs from the return fluid control in another embodiment by providing an additional small-sized or small-caliber pressure limiting valve which is actuated against the force of a closing spring by the pressure in the bypass-line downstream said fixed orifice restriction. If in this case the consumer load in the actuator or consumer feed line exceeds the pressure limit set by the rating of the spring of the pressure limiting valve, this valve opens to dump fluid from the actuator feed line through the fixed orifice restrictor to the reservoir. Consequently the pressures at the piston of the 2-way, 2 position valve interposed in the return line change and the resulting pressure imbalance will cause the valve to open, thus bleeding excess fluid in the actuator feed line back to the reservoir return.
One modification ensures through uncomplicated changes in the circuitry that the 2-way, position valve also opens when the force of the fluid in the return line exceeds the fluid pressure in the consumer feed line by an amount corresponding to the setting of the spring. In this way it is possible to ensure a repleneshing or make-up fluid supply function that helps to protect the valve system against cavitation.
Useful improvements of the valve system assure reliability in load sensing. One modification offers the particular advantage that the amount of fluid necessary to replenish both the load sensing line which is connected to the pump and the compensators at the moment of actual load sensing will be supplied by the pump and not by the load, thus effectively preventing a load from falling during the lifting cycle.
Yet another measure to supply make-up fluid to the load sensing line connected to the pump from the actuator or consumer feed line is provided by the invention.
For a better understanding of the present invention, reference is made to the following description and accompanying drawings while the scope of the invention will be pointed out in the appended claims.